This invention relates to switches and particularly to resilient switch control devices which provide a tactile sensation to the operator as the switch is depressed.
Individual key switches and keyboards (having multiple switches) are used in such diverse products as data input terminals, typewriters, computers, appliances, cash registers, calculators and electronic games. Individual keys typically have inscribed thereon alphanumeric characters or other symbols. When depressed by the operator typically a circuit is closed which provides an output which may be stored or which causes performance of a particular operation. Various spring control devices are used to ensure that a depressed key returns to its original or neutral position after release. In addition to the traditional use of helical springs for this purpose, there has been an increased use of elastic materials to provide the needed return force.
The desirability of designing the elastomeric spring control device to provide the operator with a tactile feel at or prior to the switch triggering or makepoint is recognized as being desirable, particularly in the so-called full travel keyboards. In this manner a steep dropoff of reaction force results as the elastomeric material undergoes a snap transformation confirming to the operator that positive contact has been achieved. Typical prior art elastomeric control devices which provide at least some tactile feedback to the operator include U.S. Pat. Nos. 3,478,857 to Linker; 3,603,756 to Carpentier et al; 3,829,646 to Lorteije et al; 3,932,722 to Obata et al; 4,127,752 and 4,127,758 both to Lowthorp; 4,354,068 to Sobol; and 4,362,911 to Sears et al. In addition, U.S. Pat. No. 4,289,943 to Sado is pertinent however according to the invention of that patent the elastomeric control device is depressed for a relatively short distance before the switch triggering point is made. With a further pushing force the circuit is opened and it is only then that the switch control device undergoes a snap transformation, reportedly for the purpose of reducing "bounce" and "chatter".
Despite the tactile feel offered by these prior art devices, they have not provided a sufficiently steep drop-off in force at the breakover point at snap to simulate a highly successful and commercially available QWERTY keyboard which utilizes effective but quite expensive electromechanical switch controls.
Moreover, most of the prior art devices do not provide sufficiently long pretravel of the key prior to the breakover or snap point for failsafe operation. This has resulted in an overly touchy feel in which the operator can inadvertently trigger the switch.
Among the objects of the subject invention are to overcome the aforementioned drawbacks and specifically to provide a switch control device which is characterized by a long pretravel prior to snap; in particular to shift the force displacement curve such that the snap or breakover occurs after approximately the half-way point in total key displacement; to preferably increase the tactile feel sensed by the operator by providing a substantially increased incremental drop in force per unit of displacement, after the breakover or snap point; to provide a return force/displacement characteristic which has a relatively low force level that does not push as strongly against the operator's finger during return of the depressed key; and in general to provide a relatively low cost, long life reliable switch control device and switching mechanism which is easy to produce from available moldable elastomeric materials and which offers selectivity in operational characteristics by adjustment of different spring component portions of the control device, and which preferably tends to self-center even when subjected to off-center applied force.